This application relates to quantum-well radiation detectors, and more specifically, to radiation coupling to quantum-well photodetectors.
Quantum-well structures can be used to construct photodetectors to detect radiation with a high sensitivity. One type of quantum-well photodetectors use light absorption by transitions between different quantum energy states within the same band, either the conduction or the valance band of their quantum-well structures. Such transitions within the same band are also referred to as intersubband transitions.
Quantum physics dictates that an intersubband transition is excited to absorb photons of input radiation only when the electric field of the photons (i.e., the polarization) has a component along the growth direction of a quantum-well structure, i.e., perpendicular to the quantum layers. A coupling mechanism is often implemented in such quantum-well detectors to achieve the above-desired coupling condition. Random reflectors, corrugated surfaces, and grating couplers have been used to convert normally incident radiation to waves have components that propagate along the quantum well layers.
The present disclosure includes techniques for coupling radiation to a quantum-well detector based on intersubband transitions by using a special grating coupler. In one embodiment, this special grating includes a two-dimensional array of periodic grating cells. Each cell is shaped to define at least three different grating directions.
A central region and a peripheral region of different thickness values may be included in each cell to reduce the amount of energy in the zeroth diffraction order of the grating coupler. The peripheral region conforms to and surrounds the central region. The difference in optical thickness of the central and peripheral regions may be about one quarter of a selected radiation wavelength. In addition, the areas of the central and peripheral regions may be substantially the same.
This grating coupler may be formed directly over the quantum-well detector so that the array of the grating cells is substantially parallel to the quantum well layers of the detector. A conductive contact layer may be formed over the grating coupler is electrically biased relative to another contact layer formed on the other side of the quantum well layers.